Pd/V2O5 device for colorimetric H2 detection

ABSTRACT

A sensor structure for chemochromic optical detection of hydrogen gas over a wide response range, that exhibits stability during repeated coloring/bleaching cycles upon exposure and removal of hydrogen gas, comprising: a glass substrate ( 20 ); a vanadium oxide layer ( 21 ) coated on the glass substrate; and a palladium layer ( 22 ) coated on the vanadium oxide layer.

This application claims priority from U.S. Provisional Application Ser.No. 60/202,153, filed May 5, 2000.

CONTRACTUAL ORIGIN OF INVENTION

The United States Government has rights in this invention under ContractNo. DE-AC3699GO10337 between the United States Department of Energy andthe National Renewable Energy Laboratory, a division of the MidwestResearch Institute.

BACKGROUND ART

The invention relates to an ultra-stable vanadium oxide thin filmstructure for the detection of hydrogen gas. The hydrogen gas isdissociated on the Pd catalyst into H atoms, and the V₂O₅ layer on whichthe Pd is coated functions as a H⁺ insertion host. The Pd layer is thusstabilized, which upon combination with hydrogen is chemochromicallychanged.

Hydrogen is a plentiful, clean, non-polluting fuel. Hydrogen iscurrently used in many industries, and the US demand for hydrogen isapproximately 140 billion cubic feet per year and growing. However,hydrogen is explosive at 4% in air. Therefore, it is critical tomeasure, monitor and control hydrogen wherever it is used.

In the gas detection art where sensors and measurement instrumentationfor hydrogen gases detect and/or measure hydrogen, typically there isrequired a portable sensing device, a kit (where hydrogen gas detectionand/or measurement is required in existing equipment), and sensor headsinstalled at points where hydrogen leaks are possible, or wheremonitoring is necessary (i.e., in internal combustion engines whichoperate using hydrogen as a fuel).

The problems associated with current H₂ detection devices is that thesedevices do not exhibit stable cycling during repeated coloring/bleachingprocesses and are encumbered by a narrow response range for detectingH₂.

DESCRIPTION OF THE RELATED ART

At present, H₂ detection may be accomplished through the use of variousand sundry devices, including thin film Pd oxide devices. However,several problems or drawbacks are associated with the use of thesehydrogen detecting devices. These problems are: they do not exhibitstable cycling during repeated coloring/bleaching processes; and theyare encumbered by a narrow response range for detecting hydrogen.

Inadequate cycling stability during repeated coloring/bleachingprocesses and the narrow response range for detection of hydrogen gas,in the case of the Pd thin film is due to the fact that, in the presenceof high concentrations of H₂, palladium hydride is formed and the sensoris destroyed.

DISCLOSURE OF THE INVENTION

One object of the present invention is to provide an ultra-stablepalladium vanadium oxide film structure for chemochromic detection ofhydrogen.

Another object of the present invention is to provide an ultra-stablepalladium vanadium oxide structure for chemochromic detection ofhydrogen that exhibits stable cycling during repeated coloring/bleachingprocesses.

A further object of the present invention is to provide an ultra-stablevanadium oxide film structure for chemochromic detection of hydrogen inwhich the proton insertion material contributes to stabilize thepalladium layer, and exhibits a wider response range for detecting H₂.

In general, the invention is accomplished by providing apalladium/vanadium oxide layer sensor device in which, a V₂O₅ thin filmis coated on a transparent or glass substrate. Thereafter, a palladiumlayer is evaporated onto the V₂O₅ thin film. The palladium layer servesas a catalyst material that facilitates reaction with hydrogen gas. Thatis, the hydrogen gas is dissociated on the Pd catalyst into H atoms,which diffuse into the V₂O₅ film

The vanadium oxide layer acts as a hydrogen insertion host while thepalladium layer is responsible for optical modulation. The presence ofan ion storage host is vital to the stability of the palladium layer,and the sensor formed therefrom exhibits a wide response range fordetecting hydrogen and shows very stable cycling during repeatedcoloring/bleaching processes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a hydrogen sensor comprising SiO₂ deposited on a glasssubstrate, with Pd in turn deposited on the SiO₂ layer in which theinterface between the SiO₂/Pd layer blocks the H₂.

FIG. 1B shows a hydrogen sensor in which V₂O₅ is deposited on a glasssubstrate, with Pd in turn coated on the V₂O₅ layer in which the Pd/V₂O₅interface does not block H₂.

FIG. 2 is a graph showing percent relative transmission versus time fora 20 nm VO_(x)/30 nmPd sensor in the presence of 4% hydrogen.

FIG. 3 is a graph showing percent relative transmission versus cyclenumber as a measure of cycling stability for a 20 nm VO_(x)/30 nmPdsensor in an environment of 4% H₂.

DESCRIPTION OF THE INVENTION

Due to the fact that Pd/WO₃ sensors are saturated in the presence ofjust 2% H₂, and the fact that Pd sensors in the presence of H₂ formpalladium hydride that results in destruction of the sensor, there is aneed in the interest of safety to provide a H₂ sensor that still signalsa chemochromic response to H₂ over a wide response range and in excessof 2%.

A further need exists in the art of chemochromic detection of hydrogenfor a sensor that exhibits stable chemochromic cycling during repeatedcoloring/bleaching processes upon exposure and removal of H₂.

The Pd/V₂O₅ chemochromic hydrogen sensor is capable of providing aresponse above the narrow range of 2% hydrogen because, unlike WO₃, thePd/V₂O₅ is not saturated at 2% H₂ or higher.

While not wishing to be bound by any theory as to why the Pd/V₂O₅ sensoris capable of functioning beyond the H₂ saturation point compared to aPd/WO₃ sensor, it is nevertheless believed that, the Pd/V₂O₅ sensorstructure does not change the thermodynamics of the system, i.e., iffully equilibrated, the Pd still forms a hydride; however, when akinetically steady state is achieved, the sensor still has the capacityto detect high concentrations of hydrogen even in a higher than normalatmospheric pressurized hydrogen atmosphere.

Reference is now made to FIG. 1A in which there is shown a sensorcomprising a glass substrate 10 on which is coated SiO₂, 11. A Pd layer12 is coated onto the SiO₂. In this sensor device, as is depicted byarrow 13 directed onto the interface designated by x between the SiO₂and Pd layers, H₂ 13 is blocked at the interface, because SiO₂ cannotreact with hydrogen. Pd hydride is formed that undergoes phasetransition at hydrogen concentration higher than 4%, resulting insubstantial volume change and failure of the sensor.

On the other hand and by contrast, in FIG. 1B in which a glass substrate20 is coated with a V₂O₅ layer 21, which in turn is coated by a Pd layer22, H₂ 23 is not blocked at the interface between V₂O₅ and Pd.Accordingly, the vanadium oxide layer acts as a hydrogen insertion hostin the Pd/V₂O₅ hydrogen sensor, while the palladium layer is responsiblefor the optical modulation.

The presence of an ion storage host is vital to the stability of thepalladium layer, and, unlike the case, when SiO₂ is used in conjunctionwith a Pd layer, the Pd layer does not peel off and is not degraded inthe presence of 2% H₂ (but actually starts forming hydride at roomtemperature in the presence of about 4% H₂).

The insertion of hydrogen in V₂O₅ is governed by the following equation:

A control experiment was performed to show that the optical response wasfrom the palladium layer.

The optical modulation is governed by the following equation:

A hydrogen sensor of Pd/V₂O₅ was then prepared in which the V₂O₅=2014 Åand the Pd=31 Å.

A cathodic optical response of 2% transmittance change is observed, andthis compels the conclusion that the Pd layer is contributing to theoptical response of the sensor, but that the V₂O₅ layer acts as anon-coloring ion storage layer and operates to stabilize the entirechemochromic hydrogen detector structure.

FIG. 2 is a graph depicting percent relative transmission versus timefor a 20 nm VO_(x)/30 nm Pd hydrogen sensor when exposed to a 4%hydrogen environment, and subsequently exposed to air.

The cycling stability of a 20 nm VO_(x)/30 nm Pd hydrogen sensor at 4%hydrogen is shown in the graph of transmission relative percent versuscycle number in FIG. 3, where excellent cycling stability is exhibitedduring repeated coloring/bleaching cycles. The difference oftransmittance between bleached and colored curves does not decrease withcycling.

From the foregoing, it is apparent that ion insertion host capability ofthe vanadium oxide layer is necessary to obtain stable cycling, andthat, in the absence of an ion insertion host (as in the case of SiO₂)control experiment stable chemochromic cycling is not obtained due tothe fact that palladium hydride is formed and the sensor is destroyed.

The Pd/V₂O₅ hydrogen sensor results show that: a proton insertionmaterial is vital to stabilizing the palladium layer, although it doesnot contribute to the optical response; that the Pd/V₂O₅ hydrogen sensoris easy to make via thermal evaporation processes; and that a wideresponse range of between 1 to 100% H₂ concentration is available fordetecting hydrogen.

1. A sensor for detection of hydrogen gas over a wide response range,that exhibits stability in excess of 2% H₂ during repeatedcoloring/bleaching cycles upon exposure and removal of hydrogen gas,comprising: a substrate; a V₂O₅ layer coated on said substrate; and a 30nm Pd layer coated on said V₂O₅ layer; wherein said V₂O₅ layer is a H₂insertion host in the Pd/V₂O₅ hydrogen sensor, and said Pd layer is anoptical modulator.
 2. The sensor structure of claim 1 wherein the formedVox/Pd sensor is characterized by a dimension of 20 nm for the V₂O₅layer.
 3. The sensor structure of claim 1 wherein the substrate istransparent.
 4. The sensor structure of claim 1 wherein the substrate isglass.